1. Field of the Invention
The present invention relates to pressure transient testing of producing hydrocarbon (oil and gas) reservoirs, and more particularly to measuring inter-reservoir cross flow rates through unintended leaks in zonal isolation cement sheaths in offset wells from a reservoir layer being tested.
2. Description of the Related Art
In oil and gas production, it is common to have multiple layers in a subsurface reservoir, containing producible amounts of oil and gas. With multiple layers it is common to employ wells, known as producing and offset wells, which are spaced from each other to produce from the different productive layers.
For evaluation, planning, and other purposes, pressure-transient tests are performed on the productive layers of the reservoir. With offset wells, there is often an adjacent reservoir layer to a tested reservoir layer, and producing wells are drilled through the adjacent reservoir layer to obtain production. The tested layer and the adjacent layer are separated by non-reservoir rocks or tight streaks with practically zero permeability, and there is no crossflow directly between such reservoir layers.
When the adjacent reservoir layer is located below the tested reservoir layer, only the offset wells are completed to produce from this adjacent reservoir. This means that offset wells are drilled deeper to reach the adjacent reservoir layer, bypassing the tested reservoir layer. Under normal conditions, the offset wells, passing through the tested reservoir layer, are isolated from the tested reservoir layer with cement sheath in the offset wells over the intervals passing through the tested reservoir. Thus, both the tested and adjacent reservoir layers are still isolated and no crossflow of fluids is expected.
However, there are situations when the cement sheaths in the offset wells over the intervals passing through the tested reservoir layer loses its integrity through what are known as unintentional leaks, and establishes some finite conductivity of fluid. This can allow the flow of fluid from the adjacent reservoir layer to the tested reservoir layer as the tested layer is subject to pressure drawdown.
With time the pressure drawdown in the tested reservoir layer increases. The differential pressure across the impermeable streak, located between the tested and the adjacent reservoir layers, can grow due to continuous production from the tested reservoir layer. The flow from offset wells through such unintended leaks interferes with accurate layer flow measurement. Such interference can cause an overestimation of the producibility of the tested layer under investigation.
Loss of integrity in offset well cement sheaths with time can contribute to substantial growth of inter-reservoir crossflow rates, which needs to be accounted for when characterizing the commercial producibility of the tested reservoir layer. In addition, remedial workover operations on the leaky offset well(s) may be warranted based on the rates of leakage.
The offset wells may in some cases be observation wells or abandoned wells in the adjacent layer rather than producing ones. The observation wells or abandoned wells, if present, are still intersecting both reservoir layers and allowing leakage of fluid from the adjacent to the tested reservoir layer when there are unintentional cement sheath leaks.